It's psychological. You could instead rotate the object head-over-heels to "simulate" reflection, and the object would then appear upside-down, but have left an

It's psychological. You could instead rotate the object head-over-heels to "simulate" reflection, and the object would then appear upside-down, but have left and right in the same position as on a mirror. However things don't normally move head-over-heels but rather by rotating left/right (thanks to gravity) so psychologically, we assume the object must have rotated left/right to get on the other side of the mirror, thus appearing reversed in that direction.